A single walled carbon nanohorn is a tubular carbon nanotube that has a horn-like sharp-pointed conical shape at the tip end, and is composed mainly of a carbon atomic plane with graphite structure in the same manner as a carbon nanotube. This single walled carbon nanohorn is generally produced in the form of a so-called dahlia-like carbon nanohorn aggregate in which a number of single walled carbon nanohorns are aggregated in the form of a sphere with a diameter of about 80 to 100 nm and with the conical tip portion of each carbon nanohorn being disposed on the outer side. The carbon nanohorn aggregate has a very large surface area, and large quantity synthesis thereof with high purity is easy, and therefore it is expected to be useful as an adsorptive material or the like which is lightweight and low in cost (Patent Document 1 (JP-A-2002-159851) and Patent Document 2 (JP-A-2002-326032)).
On the other hand, in order to solve energy problems or environmental problems, there recently are expectations that methane, an ingredient of natural gas, will become an alternative to fuels such as coal or petroleum, and various storage methods for methane have been proposed. For example, various carbon adsorptive materials such as activated carbons, activated carbon fibers and activated carbons with high specific surface area, metal complexes and the like are prospective methane adsorbing materials.
As a carbon adsorptive material for adsorbing methane, a single walled carbon nanohorn (SWNH) is suitable because of the characteristics described above. SWNH can adsorb methane in higher density than other carbon materials, and thus is expected to be an excellent methane adsorbing material. However, at present, it is just about at or slightly below the standards for practical application (U.S. Department of Energy: 35 atmospheric pressure, 150 v/v). Incidentally, as shown in FIG. 6, by heating SWNH (squares (□) in FIG. 6 (heating to 693 K)), it is possible to increase the amount of adsorbed methane compared with unheated SWNH (circles (◯) in FIG. 6 (303 K)). In this case, however, though the amount of adsorbed methane is larger than the triangles (Δ) of FIG. 6 (A20: an activated carbon fiber (303 K)) or the inverted triangles (∇) (AX21: an activated carbon with high specific surface area (303 K)), a significant difference in the amount of adsorbed methane is not observed compared with the lozenges (⋄) (A5: an activated carbon fiber (303 K)), and further improvement of the amount of adsorbed methane has been desired.
Accordingly, this invention has been made in view of the above-mentioned circumstances, and an object of this invention is to provide a single walled carbon nanohorn adsorptive material, which solves the problems of the prior art and is useful as a novel methane adsorbing material that adsorbs much methane and can store methane.